In Orthogonal Frequency Division Multiplexing OFDM, which is used particularly in WLAN radio networks, such as those functioning in accordance with the IEEE 802.11 Standard as well as for HiperLAN, a method is used in which simultaneously a number of carrier frequencies, also simply referred to as carriers, are employed for the transmission of a digital signal. However, these carrier frequencies are only modulated with a reduced transmission rate in relation to the overall transmission rate available (across all carriers).
For this purpose, the frequency band available for OFDM is subdivided into a number of (sub)carrier bands. The carrier frequency spacing is governed by the transmission rates.
An OFDMA-based access scenario in a system with a number of users (Multiple User system) is based on the approach of assigning each of the users their own OFDM sub-carrier.
In a system of this type and under real transmission conditions crosstalk effects between the carriers arise, also referred to as ICI (Inter-Channel Interferences).
ICI is produced in this case both as a result of a doppler shift arising from the movement of mobile terminals and also as a result of an oscillator phase noise.
In an OFDM system, the so-called “downlink,” which in mobile communications generally identifies communication going from a base station to a mobile station, both the doppler shift as well as the part of the oscillator phase noise generally corrected/compensated in the receiver, which is called Common Phase Error (CPE), is the same for all carrier frequencies of the sub-carrier bands, so that for this communication direction no access problem triggered by the OFDMA principle arises.
A system and a method is known here from US 2002/0105901 A1 in which, by forming the signal waves, a manipulation of the spectrum of an OFDM signal is achieved.
From Staamoulis et al: “Space-time block codes OFDMA with linear preceding for multirate services,” IEEE Transactions on Signal Processing, January 2002, a system is known which eliminates a multi-user interference in an OFDMA system.
From EP 0 938 208 an OFDMA/TDMA system is known with a number of users, in which sub-carriers in edge areas of a sub-carrier band are not modulated, in order to eliminate interference on adjoining frequency slots.
In the “uplink,” a term generally used in mobile communication to designate the communication going in the opposite direction, from a mobile station to a base station, the problem arises of the doppler shifts not being constant over all sub-carriers as a result of the different relative speeds of the mobile subscribers. In addition, the phase noise or the correctable part of the phase noise for this communication direction is uncorrelated as a rule since it is predominantly generated by the unsynchronized oscillators of the individual users.
The Inter Channel Interference produced by the doppler shift as well as the phase noise with his/her communication direction represents, in an OFDMA-based uplink a limitation of the transmission characteristics which can go as far as resulting in a complete failure of the system.
Accordingly, the present invention seeks to specify a method as well as an arrangement which make possible an essentially interference-free OFDMA access in the uplink.